"That said ... if we can't get Assad to yield to the pressure that we are all bringing to bear, we may have to consider additional measures."
She declined to elaborate on what those measures might be.
The official comments on Tuesday followed a cautious assessment from General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, who told CNN over the weekend that Washington still did not know enough about Assad's opponents.
"Until we're a lot clearer about who they are and what they are, I think it would be premature to talk about arming them," General Dempsey said.
Syrian government forces killed more than 60 people on Tuesday in assaults on villages and an artillery barrage on the restive city of Homs, activists said. The Red Cross called for daily ceasefires to allow in urgently needed aid.
The United States and its allies hope this week's Tunis meeting will allow them to begin drawing up a plan for Syria after Russia and China vetoed a Western-backed Arab League peace plan at the UN Security Council.
US officials suggest the meeting will focus on ways to increase economic pressure on Assad through additional sanctions and to ramp up humanitarian relief for victims of the repression.
But Arab diplomats have suggested that formal or informal moves to arm the rebels may also be discussed.
Some US politicians such as Republican Senator John McCain support efforts to arm the Syrian rebels – if not directly by the United States, then by other countries or third parties.
"There are ways to get weapons to people who are fighting against this kind of oppression, we showed that in Libya," Mr McCain told reporters on a visit to Jerusalem.
"To somehow sit by and watch this massacre continue without exploring and employing every option that we possibly can to stop it is a betrayal of everything the United States stands for and believes in."
With both Russia and Iran firmly backing Assad's government, political analysts say tacit U.S. support for arming rebel fighters could be risky given Syria's complex ethnic and religious make-up and strategically important position.
"Force employed by the Friends of Syria should be the last step of an escalatory ladder," Robert Danin, a Middle East expert at the Council on Foreign Relations, wrote in an opinion piece on Tuesday.
"Arming the Free Syrian Army and other opposition groups may eventually help topple Assad, but it also increases the potential for a fractured or failed state."